The goal of this research is to define biochemical mechanisms of avian retrovirus replication through the study of interactions of viral proteins with nucleic acids. The proposal outlines experiments 1) to elucidate the mechanism of specific cleavage of cloned viral LTR DNA by the endonuclease associated with reverse transcriptase (RT-endonuclease) and to evaluate its potential role in viral DNA integration and 2) to study the interaction of the viral pp12 nucleocapsid protein with viral RNA, evaluate its role in virion assembly and its ability to act as an RNA unwinding protein during reverse transcription of viral RNA. RT-endonuclease: Specific experiments are outlined 1) to define the nucleotide sequence at the U5/U3 junction sequence of two tandem LTRs required for specific cleavage. This will be accomplished by mapping endonuclease cleavage in single-stranded and RF I DNAs that contain deletions of sequence about this primary LTR cleavage site and by reconstruction of the site by direct synthesis (18). 2) An exonuclease activity, which has been detected in all of our purified reverse transcriptase preparations, will be characterized and its potential role in integration evaluated. 3) The action of the RT-endonuclease on cell DNA sequences known to contain sites for integration of retrovirus DNA will be examined. 4) The knowledge gained from the above studies will be used to attempt to reconstitute a non-homologous viral - cell DNA recombination system using cloned DNAs. pp12: The role of phosphorylation of serine residues in pp12 and the ability to alter the binding of this protein to viral RNA will be further studied. The specific residues that are phosphorylated in vitro and in vivo will be identified and the action of protease activated kinase I to phosphorylate pp12 will be studied. Protein phosphatases, capable of releasing phosphate from pp12, will be purified from extracts of virus free of contaminating nuclease activities. Then their ability, in conjuction with pp12, to stimulate elongation synthesis by reverse transcriptase primed with viral RNA in vitro will be investigated.